Decomposition of the refuse within a sanitary landfill produces landfill gas which contains methane. In some landfills the methane concentration is sufficient to warrant recovery of the landfill gas so that the methane can be utilized. In some instances, the methane is separated from the other constituents of the landfill gas by an adsorption process, and in other instances, the methane is used without removal of the impurity.
To recover the landfill gas, a well is sunk in the landfill. The chemical reaction within the landfill creates a pressure greater than atmospheric so that the landfill gas migrates into the well. However, to augment this natural flow, a pump can be used to reduce the pressure in the well below ambient.
From the standpoint of maximizing production of landfill gas, it would appear desirable to withdraw the landfill gas from the well at the maximum rate at which the pump is capable of operating. However, if this is done, air may be drawn into the landfill through the surface of the landfill. Air entry into the landfill is acceptable if the object of the well is simply to remove the methane to avoid the hazard presented by its combustible properties. However, if the purpose of the well is to recover the methane for use, air entry into the landfill is totally intolerable because the oxygen poisons the micro-organisms which are essential to the production of methane by the landfill. Accordingly, air entry must be avoided.
One attempt to avoid air entry involves withdrawing the landfill gas and making periodic analyses of its composition. If the gas has a predetermined oxygen content, the withdrawal rate is reduced in an effort to eliminate the oxygen in subsequently taken gas samples. The primary problem with this method is that it indicates air entry only after air has entered and flowed through portions of the landfill to the well. In other words, no indication of a problem is provided until the oxygen poisoning is well underway. There is also a lag between taking corrective action following oxygen detection and discontinuance of air entry into the landfill. For these reasons, the compositional analysis method is inadequate to avoid the oxygen poisoning problem.
Thus, a primary problem in recovering the methane from landfills is the selection of a withdrawal rate which is commercially feasible and which avoids oxygen poisoning. A related problem is determining the spacing between adjacent wells in the landfill. Proper well spacing is important to assure that all zones of the landfill are subjected to the influence of a well without causing air entry.